SCAVENGE FIRES | Cause, indication, Action and prevention

Scavenge Fire

For a scavenge fire to occur there must be present combustible material, oxygen or air to support combustion and source of ignition.

The combustion material for scavenge fire is oil.

• It can be cylinder oil which is drained from cylinder spaces,
• crankcase oil carried upwards due to faulty stuffing box or
• fuel oil from defective injectors, injectors with incorrect pressure settings,

The oxygen necessary for combustion comes from scavenge air which is available in abundance.

CAUSES :

The source of heat for ignition can be due to piston blow past which may be due to 

• sticking or broken piston rings or
• excessive liner wear.
• Faulty combustion due to late injection or incorrect atomization may also be responsible as it will cause blow back of exhaust through the scavenge ports.

INDICATIONS : 

• Loss in power and irregular running of the engine
• High exhaust temperature of corresponding unit
• High local temperature in scavenge trunk.
• Surging of turbochargers
• Sparks and smokes emitted from scavenge drains
• External indication will be given by smoky exhaust and soot and carbon particles in the exhaust.
• In modern UMS ships, temperature sensors are fitted which will activate alarm and cause automatic slow down of the engine.

ACTION :  If a scavenge fire is detected the immediate objective is to contain the fire within the scavenge space of the engine and to prevent or minimize the damage to the engine.

• Reduce the speed of the engine
• Cut off fuel to affected cylinder.
• Increase the cylinder lubrication of the affected cylinder to prevent seizure.
• Close all the scavenge drains to prevent the discharge of sparks, smoke in the engine room.
• Personnel should stay clear of the scavenge relief doors.

A minor fire will shortly burn out without damage, and conditions will gradually return to normal. The affected units must be run on reduced power until inspection of scavenging trunking and overhaul of cylinder and piston can be carried out at earliest opportunity. The actual cause of fire should be investigated.

If the scavenge fire is a major one or if there are chances of fire spreading to the adjacent spaces,

• engine should be stopped, normal cooling should be maintained
•  turning gear should be engaged and engine turned or turning gear.
•  Fire extinguishing medium should be applied through the fittings in the scavenge trunk. This may be CO2, dry powder or smothering steam.
• Boundary cooling of the scavenge manifold may be necessary.
• Scavenge manifold should not be opened until the engine has cooled down.

PREVENTION :

• Scavenge trunk must be periodically inspected and cleaned. Any build up of contamination to be noted and rectified.
• To carry out inspection through scavenge ports at regular interval and monitor the condition of piston, piston rings and liner.
• Scavenge drains should be cleaned regularly and ensured that they are clear.
• Piston rings must be properly maintained and lubricated adequately so that blow by is prevented.
• Ensure cylinder lubrication timing and quantity is correct.
• Fuel injection equipment to be maintained in good condition.
• If the cylinder liner wear has reached its maximum limit the possibility of scavenge fire will not be reduced until the liner is renewed.

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Author marineGuru

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SUPERCHARGING IN MARINE DIESEL ENGINE

Supercharging (or Pressure charging) is a process where a greater mass of air is admitted in the cylinder for combustion and consequently a greater amount of fuel is burnt efficiently.
Power output of the engine is increased with higher thermal efficiency without increasing the size of the engine.

Naturally aspirated engines : It is a term applied to engines where air charge is brought into cylinder only by the downward movement of the piston, without any external aids.

Supercharged engines : is the term used to indicate that weight of air supplied to the engine is considerably increased.
Supercharger can be any device (normally exhaust gas driven turbocharger) which increases the pressure of combustion air supply above that which is normally required.

Air Charge Ratio : This is ratio of volume of air contained in the cylinder at the the start of compression to the swept volume of the piston. For naturally aspirated engines it is about 0.85 where as for supercharged engines it is about 2.5 – 4.0.

Advantages of pressure charging :
1. Substantial increase in power for given speed and size of the engine.
2. Better power to weight ratio i.e. reduced engine weight for given output
3. Improved mechanical efficiency with reduction in specific fuel consumption
4. Reduction in cost per unit of power developed.
5. Increase in air supply has a considerable cooling effect leading to reducing the  severe working conditions and improved reliability.

There are 2 types of Supercharging methods :
Pulse type  and
Constant pressure type.

Supercharging Techniques

PULSE SYSTEM : IN PULSE PRESSURE SYSTEM THE ENERGY OF THE EXHAUST FROM THE CYLINDER IS TRANSFERRED TO TURBOCHARGER BY PRESSURE WAVES OR PULSES.

Pulse Turbo-charging System with Twin entry Blower

• THESE WAVES TRAVEL THROUGH THE MANIFOLD TO THE TURBINE NOZZLES WHERE THEY ARE CONVERTED TO KINETIC ENERGY AT HIGH VELOCITY TO ROTATE THE TURBINE BLADES.
• THIS GIVES A RAPID BUILDUP OF TURBINE SPEED WHEN AN ENGINE IS STARTED OR DURING MANOEURING.
• TO MAINTAIN THE PULSES, RAPID OPENING OF EXHAUST  VALVE, EXHAUST CONNECTIONS OF LIMITED DIAMETER, NO SHARP BENDS & THE TURBOCHARGER IS FITTED CLOSE TO THE ENGINE.
• TO PREVENT BACK FLOW INTO  THESE, THE EXHAUST IS SUBDIVIDED BETWEEN A NUMBER OF MANIFOLDS, EACH CONNECTED TO A SEPARATE NOZZLE BOX AT THE TURBINE.
• UPTO THREE CYLINDERS USE EACH MANIFOLD WITHOUT INTERFERENCE DEPENDING ON THE FIRING ORDER.
• DUE TO SMAL VOLUME OF EXHAUST DUCTING AND DIRECT LEADING OF EXHAUST TO TURBINE INLET THE SYSTEM DOES NOT REQUIRE ANY FORM OF SCAVENEGE ASSISTANCE AT LOW SPEEDS OR WHEN STARTING.

ADVANTAGES :
UTILIZES HIGH KINETIC ENERGY OF EXHAUST GASES
DOES NOT REQUIRE AUX BLOWERS AT LOW SPEEDS OR DURING STARTING OF THE ENGINE.

DISADVANTAGES :
MORE NO OF TURBOCHARGERS NEEDED
EXHAUST PIPING IS KEPT STRAIGHT AND SMALL SO LOCATION OF TURBOCHARGER IS VERY CRITICAL
ACCURATE MATCHING OF EXHAUST REQUIRED TO PREVENT BLOWBACK.
CONSTANT PRESSURE : IN THIS SYSTEM THE EXHAUST GASES FROM EACH INDIVIDUAL CYLINDER IS LED TO A COMMON MANIFOLD OR RECEIVER WHERE THE PULSE ENERGY IN LARGELY DISSIPATED.

THE PRESSURE PULSE IS DAMPED OUT BY EXPANDING THE GAS IN THIS CHAMBER WHICH IS MAINTAINED AT CONSTANT PRESSURE.THE EXHAUST GAS FROM MANIFOLD IS LED TO TURBOCHARGER AT CONSTANT PRESSURE.

THE VOLUME OF THE MANIFOLD MUST BE LARGE ENOUGH TO ACCOMMODATE GAS FLOW AND PREVENT LOCALISED PRESSURE RISE.

ADVANTAGES :
1. STEADY PRESSURE BEFORE TURBINE,EFFICIENT OPERATION
2. BETTER AND MORE RATIONAL UTILISATION OF EXHAUST HEAT
3. COMPRESSOR CAPACITY CAN BE INCREASED AS MORE ENERGY IS AVAILABLE
4. EXHAUST PIPING IS SIMPLER
5. EXPANSION IN CYLINDER CAN BE CARRIED OUT LONGER AS NO PRESSURE PULSE IS REQURED.THIS RESULTS IN EFFECTIVE INCREASE IN STROKE SO MORE OUTPUT.
6. DUE TO EFFECTIVE SCAVENGING SFC CAN BE REDUCED

DISADVANTAGES :
THIS SYSTEM IS SLOWER IN ITS BUILDUP OF PRESSURE WHEN STARTING, & INSUFFICIENT AIR IS AVAILABLE FOR ENGINES DURING MANOEUVRING OR OPERATING AT LOW SPEED.
TO OVERCOME THIS DIFFICULTY ENGINES HAVE ELECTRIC DRIVEN AUXILLARY BLOWER.

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Author marineGuru

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